1. Field of the Invention
The present invention relates generally to the field of leather tanning and more particularly to a formulation and process using chromium III salts with polymer complexing agents to tan leather.
2. Description of the Prior Art
The tanning of animal hides using various chemicals to produce leather and other products is known in the art. Animal hides must be treated in order to preserve them from decay since they contain a high content of degradable protein, as well as water and other matter. To prevent decay, hides are subjected to various treatments.
The hides are usually first subjected to processes such as liming, de-liming, bating and pickling. In addition, an oxidation step can be employed for whitening the hides using agents like sodium chlorite (or other bleaching agents). Pickling is done using mineral salts like sodium chloride in sulphuric acid and possibly formic acid.
The actual tanning step consists of treatment by ether vegetable extracts or mineral tanning agents. One very common type of mineral salt tanning is to subject the hide to a treatment employing a chromium III sulfate or chloride which, under acidic conditions, penetrates into the leather structure. The chromium salts form a complex with the carboxyl groups of protein during the subsequent step of basification. Chrome treatment is known to impart superior chemical and physical properties to the leather than tanning with vegetable agents.
The tanning process normally consists of the steps of treating the hide with chromium salts, basification where cross-linking of the chromium with the carboxyl group of the collagen occurs, re-chroming where with more chromium salts to improve chromium fixation and re-basification. The product is than neutralized to PH 4.5–5.6 to make it ready for the dyeing and other processes. Chrome tannage involves a cross-linking reaction between carboxyl groups of the collagen molecules and the chromium III salt. The carboxyl groups, when ionized, are attracted to the chromium III tanning complex, and a reaction takes place. The following diagram shows a possible structure for the chrome/hydroxyl/sulfate complex.
